WO-A-02070627 describes a process for preparing a base oil having a kinematic viscosity at 100° C. of 22 cSt from a heavy Fischer-Tropsch wax.
A problem of the prior art processes is that especially the base oils having a high viscosity often show a haze. This haze makes the process less suitable for some applications. However not all applications for this family of base oils require that a haze should be absent.
WO-A-03033622 describes a process wherein a haze free base oil is prepared from a Fischer-Tropsch product by removing the heaviest fraction, containing the haze precursors, by deep-cut distillation.
A disadvantage of the process according to WO A-03033622 is the deep cut distillation performed at a cut-off temperature of between 1150 and 1350° F. (621-732° C.). This is not only a technically difficult distillation step it also removes the valuable heavy base oil molecules together with the haze precursors.